The spread of AIDS and the ongoing efforts to control the responsible virus are well-documented. One way to control HIV is to inhibit its reverse transcriptase activity (RT). Thus, novel, potent, and selective inhibitors of HIV RT are needed as useful therapeutic agents. Known, potent inhibitors of HIV RT include 5'-triphosphates of 2',3'-dideoxynucleoside ("ddN") analogues. These active RT inhibitors are generated intracellularly by the action of nucleoside kinase and nucleotide kinase. Thus ddN compounds such as AZT and d4T have been considered to hold much promise in the search for anti-HIV agents.
The rate-limiting step for the conversion of 3'-azido-3'-deoxythymidine (Zidovudine; AZT) to its bioactive metabolite AZT-triphosphate seems to be the conversion of the monophosphate derivative to the diphosphate derivative, whereas the rate-limiting step for the intracellular generation of the bioactive 2',3'-dideoxy-2',3'-didehydrothymidine (d4T) metabolite d4T-triphosphate was reported to be the conversion of the nucleoside to its monophosphate derivative.(Balzarini et.al., 1989, J.Biol. Chem. 264:6127; McGuigan et.al., 1996, J. Med. Chem. 39:1748). See FIG. 1 for the mechanism proposed in the prior art.
In an attempt to overcome the dependence of ddN analogues on intracellular nucleoside kinase activation, McGuigan et al. have prepared aryl methoxyalaninyl phosphate derivatives of AZT (McGuigan et.al., 1993 J. Med Chem. 36:1048; McGuigan et.al., 1992 Antiviral Res. 17:311) and d4T (McGuigan et.al., 1996 J.Med.Chem.39:1748; McGuigan et.al., 1996 Bioorg.Med.Chem.Lett. 6:1183). Such compounds have been shown to undergo intracellular hydrolysis to yield monophosphate derivatives that are further phosphorylated by thymidylate kinase to give the bioactive triphosphate derivatives in a thymidine kinase (TK)-independent fashion. However, all attempts to date to further improve the potency of the aryl phosphate derivatives of d4T by various substitutions of the aryl moiety without concomitantly enhancing their cytotoxicity have failed(McGuigan et.al., 1996 J.Med.Chem.39:1748).
In the present invention, it has been discovered that a substitution at the phenyl moiety in the phenyl methoxyalaninyl phosphate derivative of d4T with an electron-withdrawing moiety such as a para-bromo substitution, enhances the ability of the phenyl methoxyalaninyl derivative of d4T to undergo hydrolysis due to the electron withdrawing property of the bromo substituent. The substituted phenyl phosphate derivative of d4T demonstrate potent and specific anti-viral activity.